- Fertilizing Rhododendrons: How And When Do You Fertilize Rhododendrons
- Best Fertilizer for Rhododendrons
- Rhododendron Fertilizer Schedule
- How to Fertilize Rhododendron Bushes
- Fertilising Rhododendrons
- Give rhododendrons and azaleas an annual check-up each spring
- Gardening Answers Knowledgebase
- Growing Rhododendrons
- JARS v56n2 – Tips for Beginners: Fertilizing Rhododendrons
Fertilizing Rhododendrons: How And When Do You Fertilize Rhododendrons
Rhododendrons are one of the first flowering shrubs to bloom in the spring. The popular bushes can be long-lived and healthy if given proper care. But to get the most flowering power, you may need to fertilize. So when do you fertilize rhododendrons? Read on for tips about the best fertilizer for rhododendrons and when/how to get the job done.
Best Fertilizer for Rhododendrons
If you have planted your shrubs in fertile soil, fertilizing rhododendrons is not a necessity. However, those rhododendrons growing in poor soil need feeding. Likewise, plants grown with mulch that depletes nitrogen in the soil, such as fresh wood chips, probably will do better with a regular rhododendron fertilizer schedule.
If your shrubs require feeding, you not only need to know when and how to fertilize rhododendron bushes, but you’ll want to use the best fertilizer for rhododendrons. They require a complete food at appropriate intervals.
Although you’ll find some products labeled as specific for rhododendrons and azaleas, these are not necessarily the best for your plants since they acidify the soil. You only need this if your soil is not sufficiently acidic for the plants.
Plants generally require three nutrients to thrive: nitrogen, phosphorus and potassium. Most fertilizers contain a mix of these three nutrients, with the percentages of each listed in that order on the label. A complete fertilizer contains all three of these in different percentage amounts.
Rhododendron Fertilizer Schedule
You’ll want to use a particular fertilizer composition for the different stages of your rhododendron growth. At planting time, use 10-10-6 fertilizer before you water the plant in.
In early spring, the rhododendron buds swell. At this time, apply a complete 10-8-6 fertilizer. Apply another light dose of this fertilizer when the leaves emerge. In autumn, applying organic compost or manure is sufficient.
How to Fertilize Rhododendron Bushes
You can choose between various types of fertilizer for your plants. Granular fertilizer comes in grains that are sprinkled on the soil and watered in. Liquid or water-soluble fertilizer is mixed with water and then poured into the soil.
If you are concerned about cost, granular fertilizer is the best fertilizer for rhododendrons since it is the cheapest. Liquid fertilizer passes quickly through the soil and application must be repeated more frequently.
A general rule of thumb is to use one level tablespoon of fertilizer for every foot of plant growth. You can purchase granular fertilizer that is labeled slow-release. It releases its nutrients slowly, over time.
Fertilisers for Rhododendrons and Azaleas.
Fertilising can be discussed in terms of Organic and Inorganic (man made). Both options are available to the rhodo gardener but both come with some pretty important messages. Rhododendrons require regular feeding, but their demand for food is specific. They do not like lime and if you have limechip on your driveways or pathways, lime can leach into the garden and cause them to turn up their toes.
Fertilisers assist with over plant health, they help to create a better shaped rhodo, and increase your chances of flowering. If a rhodo is under stress in the summer months when flower production takes place, the rhodo will go into survival mode and will put all its remaining energy into survival and forget about flowering.
If your rhododendron starts to turn yellow it is a strong sign that your rhodo needs feeding.
Organic Fertiliser Options
Some of the best things in life are free – and fertiliser can apply to this quote too. Rhododendrons are acid loving plants and will enjoy a regular dose of pine needles around them, pine needles are acidic and are the perfect mulch for rhodos.
You can successfully fertilise rhododendrons using organic materials however if a rhodo has turned yellow like the photo above it will be a slower recovery back from this colour to a nice green colour. Inorganic fertilisers can respond to the need for nutrients faster.
If you are going to fertilise using organic materials you need to take a holistic approach to the garden and be working on the nutrient needs of your plants around the year, not reacting to a problem and trying to create a quick fix.
You can burn and kill rhododendrons through using animal manure that is too strong, too close to the roots and too heavily applied. Organic animal manure needs to be well composted/rotted and often it helps to have the manure mixed with untreated sawdust or straw. You do need to know what you are doing, or ask a seasoned gardener, or get a wide variety of advice from rhododendron experts before making a decision to cover your rhodo garden in free poo.
Horse manure from a stable is very highly concentrated with urine and should be left to rot outside of the stable before being applied to a rhododendron garden.
Chookie poo and Pig Poo is very very strong and we advise to give this kind of free fertiliser a miss. Some gardeners will use it with success – we recommend that you don’t purchase it and if you are going to use it, then ensure that you test it and use it sparingly.
Sheep poo is a little less toxic and is a lot easier on the rhododendrons, the best is the rotted poo mixed with wool from under the shearing shed, often you can purchase this or get it for free if you are willing to help out a farmer – usually they will be delighted to have someone clean out under the shearing shed. When you apply the mix to the garden, keep it away from the direct circle that is around each rhododendron. Use the fertiliser on the outside perimetre of the rhododendrons and keep the area around the trunk and branches free of fertiliser.
Rhododendron roots are very fine and close to the surface, if you apply any kind of fertiliser on top of these fine roots the effect can be drastic. The first sign is that the leaves will go brown and crusty and look like they have been sunburnt. Once the leaves have been damaged they will not recover, however if the plant has not suffered too severe damage you will hopefully get a new crop of healthy leaves and you can either leave the damaged leaves to fall off by themselves or take them off yourself.
Inorganic Fertiliser Options
If you are happy to use inorganic options there are many available. We recommend the use of a slow release fertiliser. We sell coated balls, these slowly release nutrients into the ground over an 18 month period. So long as you follow the instructions for spreading you cannot cause damage to your rhodos like the photo of the burnt leaves above.
However!!!! We do know of a nursery worker who over applied fertiliser to a crop and killed the lot, so just remember fertiliser is like medicine, take what you need – overdosing can be dangerous.
During winter the granules will not release in very cold climates. So best to wait until spring before applying.
Give rhododendrons and azaleas an annual check-up each spring
AURORA – Rhododendrons and azaleas herald spring in Oregon. And spring is a good time to give these beautiful shrubs the care they deserve.
There are a number of things home gardeners can do to keep rhodies and azaleas long-lived and healthy, according to Jan McNeilan, retired horticulturist with Oregon State University Extension Service.
As with all health check-ups, start with a physical examination. Look for symptoms of nutrient deficiency, such as pale green or dwarfed leaves, which may indicate a lack of nitrogen. Yellow mottling along the leaf margins may indicate an excess of potassium. If the leaves are smaller and darker green than normal and the tips are dead, your shrub may need phosphorus. Yellow or yellow-white (chlorotic) leaves may mean there is too much calcium, which can be leached from sidewalks and foundations. When the soil pH is too high for acid loving rhodies, some nutrients are tied up in the soil and unavailable to the plant.
Be mindful of past weather conditions when looking at physical symptoms. Leaves can be discolored from hot sun or winter frost. Also, as the interior leaves of the plant are shaded out, they may turn yellow and drop off, which is a normal process called senescence.
When flowers fade, remove or “dead-head” the spent clusters. Be careful to snap off flower heads, and not leaf shoots. This will keep the plant from using energy to form seed heads and removes a hiding place for overwintering insects.
But don’t despair, says McNeilan. If you don’t get around to deadheading, the plant will survive your “less than perfection” gardening techniques.
Otherwise, rhododendrons require very little pruning unless branches are dead or injured. If you have older shrubs that have grown spindly, you can rejuvenate them by heavy pruning. The best time to prune these plants is soon after flowering, so not to interfere with flower bud formation for the following year.
Rhododendrons and azaleas prefer slightly acid soils. Fertilizer is best applied soon after flowering in the spring. Organic fertilizers are a bit more costly, but are released slowly and need to be applied less frequently than chemical fertilizers. Do not apply fertilizer after mid-July, or the plant may not be completely dormant by winter. Avoid the use of lime or alkaline fertilizers.
With shallow, fibrous root systems, rhododendrons and azaleas need ample water during the summer. Make sure to water them deeply. If they are planted under wide-eave overhangs, they will need water throughout the entire year. Avoid hoeing or raking too deeply around the plants. Use mulches such as sawdust, bark dust, peat moss, straw or other organic materials to inhibit weeds, provide more uniform soil temperatures and conserve moisture.
For more information about rhododendrons and azaleas, download OSU Extension’s fact sheet “Azalea and Rhododendron Care and Culture,” (FS 12).
Here’s some tips on how to plant and care for your rhododendrons:
– Plant your rhodo in the right place. They thrive in light shade in well-drained, acid soil that is fairly rich in organic matter.
– More rhodos are killed by overwatering or inadequate watering than from being improperly located. Remember to lightly water rhodos and azaleas in hot summer months as well as during dry spring and fall periods.
The planting hole should be wider than the root ball, but only slightly deeper. Check the drainage before planting. Rhodos hate to sit in waterlogged soil.
– Rhodos can be moved easily since they have compact, shallow roots, even when they are quite large. The best time is in autumn or spring. Transplanting is often a better idea than severe pruning.
– Fertilize before and after flowering with a light sprinkling of 10-8-6.
– Prune in early March instead of after blooming in May. This will produce the best results because the season for new growth will be extended by up to six weeks. Be brave. Rhododendrons are resilient shrubs and, unless diseased or weak, tend to flush back with amazing vigour.
– Established plants benefit from a feeding and top dressing in the early spring. A mixture of peat and garden compost or leaf mould spread in a thin layer at the base of the plant will help to maintain the acidity and nutrient level of the soil.
(A word of caution: Take care not to pile this mixture around the stem, or apply too thickly – rhododendron and azalea roots are particularly touchy about having their oxygen supply cut off underneath too much earth.)
– In winter, heavy, wet snow can bend and snap branches. The solution is simple. Take a broom handle or other long stick, and gently tap branches so the snow falls off. In areas where the snow accumulates over winter, the increasing depth of snow will eventually support the branches of larger rhodos.
Gardening Answers Knowledgebase
Search Results for: Rhododendron | Search the catalog for: Rhododendron
- Plant Answer Line Questions: 11
- Garden Tips: 2
- Book Reviews: 1
- Recommended Websites: 5
Azalea Society of America
Meerkerk Rhododendron Gardens
Rhododendron Species Foundation & Botanical Garden
American Rhododendron Society
Seattle Rhododendron Society
Plant Answer Line Question
Keywords: Rhododendron, Pruning shrubs
I need to know what to do with a rhododendron that has grown too big. I want to keep it, since it is a bookend to another plant. Can I cut it back, and if so, how far and when? Will it be okay and continue to bloom if I cut it back? Could you suggest something and also suggest a really good book on care, etc., for rhodies?
The American Rhododendron Society’s page on pruning should be helpful. It describes clean-up pruning, shape pruning, and rejuvenation pruning.
Seattle author Cass Turnbull’s Guide to Pruning (Sasquatch Books, 2012). Plant Amnesty, founded by Turnbull, also has information on pruning an overgrown rhododendron.
The Rhododendron Species Foundation in Federal Way is a place you can visit to get an idea of the wide range of species available. They also have general information on rhododendron care on their website.
The Miller Library has a substantial collection of books on rhododendrons, including quite a few by Pacific Northwest authors, and you can browse them or check them out to see which species and varieties might be more suitable for your garden.
Keywords: Transplanting, Rhododendron
Some friends of mine just bought an old house with a huge rhododendron up against the house. It is at least 8 feet high and probably 10 feet wide. I did not dig around and there may be multiple shrubs growing next to each other. What are the chances of moving the rhody successfully? Should it be cut way back before hand? Any particular time of year for moving it?
Fortunately, rhododendrons are very likely to succeed in being transplanted. Most experts recommend fall as the best time to transplant. Spring or late winter is second best.
The real challenge is getting a large enough rootball. A five-to-six foot plant requires a rootball of about 3 feet in diameter.
Step 1- dig a 12-18 inches deep trench around the rootball.
Step 2 – under cut the rootball to sever the roots from the underlying soil. The most important roots are the small feeder roots, not the big old ones. You can use a steel cable with a tractor or you can use a shovel and digging iron and a lot of hard work. The rootball will probably be about 8 – 12 inches deep and 3 feet in diameter.
Step 3 – tilt it on its side and slide a piece of 1/2-inch plywood under the rootball and set the plant upright. Use the plywood to move the plant to its new location. (A tarp works, too, if you can get it underneath the rootball.)
Step 4 – dig a new hole 4 feet in diameter and deep enough so that the rootball is 1 inch higher than the depth of the hole. (Slightly above grade)
Step 5 – water well and mulch around the perimeter of the plant BUT keep the mulch at least 2 inch away from the trunk of the plant.
Newly transplanted plants need some tender care and especially need to be watered regularly, but not over watered.
There were no recommendations to cut the foliage back. But it is always ok to prune out dead, dying, diseased or deranged stems. This also means you can prune out twiggy growth.
This information comes from Success with Rhododendrons and Azaleas by H. Edward Reiley (1992).
Keywords: Frost, Hardy plants, Rhododendron
A friend in Illinois has sent a photo this spring of a very healthy looking rhododendron – leaf buds fully elongated and beginning to unfurl, while the green, blunt flower buds remain unopened. The flower buds don’t look brown, diseased, frozen or injured, but they remain tightly closed, foliage bud growth preceding blooming. He says he has 6 plants doing the same this month. Possible reason?
Though we can’t diagnose plant problems by phone/email, early autumn frosts can inhibit flowering and not all buds are equally affected.
“Autumn frosts: These can lead to damage…if they either occur in early autumn or immediately after a late season warm spell. Continental climates with extremes of heat and cold are more likely to suffer sudden temperature changes than those with maritime climates…A sudden temperature drop will catch a plant before it has had a chance to reach maximum hardiness and it may suffer accordingly, even if normally perfectly able to withstand such a temperature in mid-winter…Speed of ripening varies considerably…There is also a variation in the hardiness of flower buds compared to foliage and growth buds. Commonly, flower buds may be as much as 10 F. less hardy than foliage…”
(Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 119-120)
Keywords: Rhododendron, Pruning shrubs
A friend was told that pinching out growth buds before they begin to elongate as a means of shaping young rhododendrons would only stimulate buds further down the stems that were less than 4 years old – older than that and the growth buds would no longer be viable. I cannot find any information to suggest 4 years viability of dormant buds to be true, or untrue. Can you help?
Though pinching encourages multiple branching lower down the stem, I find no reference to it being done at a particular age.
That statement indicates a younger plant, but the author then mentions several exceptions.
Here is some how-to information about pruning online:
7 Solutions to the Too-Big Rhododendron.
Keywords: Rhododendrons–Washington, Rhododendron
My two rhododendrons did not produce any blooms this year- they are healthy otherwise. Why?
I had the same problem with one of my rhodies this spring (all the others were fine), as did many other people in the Pacific Northwest. Following are the most likely causes:
NO FLOWERS, BUDS DO NOT OPEN. This is most likely to be caused by frost, either in mid-winter by the hardest frosts of the year, or in spring when the buds are swelling and about to open. Certain varieties have very frost-vulnerable swelling buds, while many species have buds which are easily destroyed even by quite mild winter frosts.
NO FLOWERS, NO FLOWER BUDS. There are several possibilities why rhododendrons may not flower freely:
- Too much shade. This is very common in North America where, in order to regulate sun and soil temperature, plants are placed in deep shade. This allows healthy, if straggly growth, but can inhibit flowering. The more light you can give a plant, the more likely it is to flower, so there is a trade-off between the need for shade and the need for light.
- The variety takes many years to flower (it does not sound like this is your situation).
- Kindness. Rhododendrons flower in order to reproduce. A contented, well-fed, well-watered well-shaded plant may not feel any need to reproduce, as it perceives no threat to its survival. Do not feed after mid-summer, as this encourages growth at the expense of flowers. Nurserymen cut down watering in late summer to stress plants into flowering the following year.
(Source: Rhododendrons: A Care Manual, by K. Cox, 1998, p. 73).
The above is corroborated in other sources, e.g. Success with Rhododendrons and Azaleas, by H.E. Reiley, 1992, p. 132-133.
Keywords: Failure to flower, Frost, Rhododendron, Master gardeners
This is the second year in a row that my rhododendron Blue Peter has flower buds but they are dry and somewhat dark and have no flowers at all. These buds are easy to deadhead. Can you help me salvage this rhododendron, which is very old, and beautiful when it blooms?
In order to get an accurate diagnosis you will need to take a sample of your plant (including both healthy and affected parts if possible) to a Master Gardener clinic. If they do not know what it is, ask them to send the sample to the pathology laboratory in Puyallup. It is best to go through Master Gardeners first so you will not be charged. If you send the sample yourself there will be a fee.
Meanwhile, several sources mention frost, drought, and “bud-blast” (unlikely in the Pacific Northwest) as potential causes of bud failure. Damaged flower buds and poor bud set: It is always most disappointing when fat, healthy looking flower buds either fail to open at all or only open a percentage of their buds, the rest being black and dead. Some rhododendrons regularly abort some or even all of their buds for no apparent reason. This may be due in some cases to a deficiency, perhaps magnesium, or to drought reports from various places give mixed results from applying magnesium (usually as Epsom salts)… By far the most usual cause of bud damage is frost. Flower buds are invariably less hardy than the rest of the plant so a really hard winter is sure to cause losses to flower buds. Early autumn frosts can damage buds that are not fully hardened off. This is a very annoying type of damage that may be overlooked and may not be noticed until the buds attempt to open in spring. Rhododendrons vary greatly in their ability to harden up enough to withstand early frost. In areas very prone to spring frosts, it is better to avoid growing plants that always burst into growth at the first sign of spring. Plants that frequently loose their first growth flush (and sometimes even their second) are liable to become stunted and rarely flower.
Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 244.
Keywords: Rhododendrons–Diseases and pests, Rhododendron, Master gardeners
My rhodies have black spot, rust. Is there a plant medicine I can put in the soil so it will get absorbed by the entire plant rather than spraying every other leaf.
I am sorry to hear about your sick Rhododendrons. You should take a take a leaf sample into a Master Gardener clinic for (free) diagnosis. I have linked a list of clinics in Snohomish County below. Their volunteers are trained in identifying plant diseases and suggesting solutions.
If you cannot get into a clinic try the HortSense webpage from Washington State University Cooperative Extension, and search the common ailments affecting rhododendrons (in the Ornamental Shrubs section).
The reason why it is vital to get an accurate diagnosis is because some fungal diseases do not have treatments that really work, such as rust, while other “leaf spot problems” are not caused by fungus at all, therefore spraying with fungicides or applying a systemic to the soil would only be a waste of time and money!
Try contacting the Snohomish County Master Gardener Clinics to see if you can bring in samples.
Keywords: Rosaceae (Rose Family), Rhododendron, Fertilizers
Is it okay to fertilize my rhodies, azaleas, and roses in September? I missed doing it in August.
Generally speaking, it is best not to fertilize your shrubs after mid-summer. The tender new growth that results is susceptible to frost, disease, and insects just at the time of year when the plant is beginning to shut down. This is also true of roses, which are even more tender and susceptible than rhododendrons and azaleas.
An article by Terri Richmond (British Columbia) on the American Rhododendron Society website, entitled Fertilizing Rhododendrons the Organic Way supports the practice of fertilizing in spring. (Keep in mind that azaleas are in the same genus as rhododendrons.)
Oregon State University Extension suggests that budbreak in spring is a good time to fertilize roses, just as new growth is beginning. Stop fertilizing in late summer. Oregon State University also weighs in on fertilizing rhododendrons (if needed, in spring shortly after flowering, and preferably with organic fertilizer).
Keywords: Transplanting, Rhododendron
Can I move my rhododendrons now, in late winter to early spring?
According to A Plantsman’s Guide to Rhododendrons by Kenneth Cox (Ward Lock Ltd., 1989), “rhododendrons are generally quite easily moved, most even in full flower and at considerable age. Size is really no problem, provided you have the means to do the digging and the moving. Obviously, the more rootball you can take with the plant the better, but usually you can reduce it considerably without too much harm being done. If you end up with a disproportionately small rootball, you can reduce the size of the top somewhat to compensate. The roots of a rhododendron generally extend to about 50% of the plant’s foliage diameter it can be far more or much less. The roots are usually less than 18 inches deep, even on a very large plant. To move a large plant, start digging quite far out from the stem, and continue towards it until you meet roots. Then dig all round underneath the rootball gently rocking the plant to ease the rootball from the soil. Watch out when lifting a plant by its main stem; it may not be strong enough to carry the weight of the rootball. The root can best be reduced by prising soil from it with a fork. A rhododendron can remain out of the ground for considerable periods if you keep frost and sun from the roots, and ensure that it receives regular watering. Heeling it into the ground, or covering the roots usually gives adequate protection. Although rhododendrons can be moved during the growing season, they will require extra watering after transplanting.”
In addition, you may find the Royal Horticultural Society’s directions on moving a mature tree or shrub helpful.
Keywords: Rhododendron, Poisonous plants, Pets
Are azaleas poisonous to cats?
Azaleas are indeed a problem for cats and other pets. See from Purdue University’s Veterinary Program.
“These ornamental shrubs aren’t commonly nibbled on but they can cause fatal heart problems in dogs, cats, and pet birds. Signs to watch for are similar to that of the yews and include weakness, fainting, salivation, difficulty breathing, vomiting, and diarrhea.”
According to the Handbook of Poisonous and Injurious Plants, 2nd ed., by Lewis Nelson et al. (New York Botanical Garden/Springer, 2007), all Rhododendron species, including Azaleas, contain grayanotoxins in their leaves. Honey made from the flower nectar would also be toxic.
Keywords: Shrubs–Care and maintenance, Rhododendron
Should you remove old blooms from rhododendrons and, if you should, which is best: to prune or snap them off?
Here is what the American Rhododendron Society says on the subject:
“It is desirable, with the large flowered rhododendrons, to remove the withered flower clusters after the blooming season. This is fairly easily done as the central axis of the cluster, usually called a truss, will break free from the plant with a quick snap of the thumb pushing on the side, or can be cut off with a hand pruner. With the smaller flowered rhododendrons and azaleas, dead-heading is labor intensive and and generally is not required.
Dead-heading is usually done to make the bush look more attractive, to reduce the prevalence of fungus and to prevent a heavy set of seed. If it is not possible to remove the old flowers, it is usually not too detrimental, but flowering the next year may be reduced.”
I have several mature rhododendrons in my own garden, and I deadhead the parts of the shrubs which are easily reachable, leaving the other areas to their own devices. For me, it’s an aesthetic choice, and I would probably do them all if I could reach and if I didn’t get very tired of the task. (It’s hard to do well with gloves since you can’t easily feel the right place to snap off the flower head, but it’s sticky work without the gloves.) I’ve never tried pruning them off, because it seems less precise (leaves a bit of a stub), but if rhododendron experts approve (as indicated above), I may just try it this year.
Keywords: Species Rhododendrons, Rhododendron
You need only take a walk through Washington Park Arboretum or peer into your own backyard to notice that May is the season of the rhododendron. It would seem that rhododendrons are native to the Northwest, the superb way they thrive both in the cultivated garden and wild forest floor. But those rhododendrons, which have become such a mainstay in Pacific Northwest flora, are relative newcomers to these parts and have been plucked by enchanted plant hunters from China and the Himalayas.
Jane Brown tells the dramatic and long history of the rhodies global travels in her recent book, Tales of the Rose Tree: Ravishing Rhododendrons and Their Travels Around the World, (Harper Collins, $36.75). In this accessible historical account, Ms. Brown tells of the legend & lore, as well as the botanical significance, of the rhododendron. She includes many fine illustrations and color plates of many notable representations of the rhododendron. In addition, she lists many of the best places to find rhododendrons, mainly in the UK, where she resides. Travel to the Royal Horticultural Society’s Wisley Garden or the Edinburgh Royal Botanic Garden for grand displays of this woody and lovely plant.
Or remain closer to home and visit The Rhododendron Species Foundation and Botanical Garden in Federal Way. Twenty-two acres encompass nearly 10,000 rhododendrons in all shapes, sizes, colors, and scents. From March through May, the Garden is open from 10:00 – 4:00 six days a week (closed on Thursdays). June through February, the Garden is open 11:00 – 4:00 five days a week (closed Thursdays and Fridays). Admission is $3.50 for adults and $2.50 for seniors and students. For additional information and directions, call: 253-927-6960.
Keywords: Sarcococca, Plant cuttings, Salvia, Lavandula, Propagation, Rhododendron, Gaultheria shallon, Penstemon, Holly, Cistus, Ceanothus
Make new plants by taking softwood cuttings. Cuttings Through the Year, a booklet published by the Arboretum Foundation(available for sale at the Washington Park Arboretum gift shop) suggests which plants to propagate month by month and how to do it. A few September plants include: Rock Rose, Salal, Lavender, Holly, Penstemon, evergreen azaleas, Sweet box, Salvia, California Lilac and many others.
For a tutorial on taking softwood cuttings go online to a Fine Gardening article complete with clear color photos: www.finegardening.com/propagate-your-shrubs-softwood-cuttings
Back in the garden with Dulcy : the best of The Oregonian garden writer Dulcy Mahar by Ted Mahar, 2013
Reviewed by: Brian Thompson on 2014-10-01
A book that took me totally by surprise is “Back in the Garden with Dulcy”, a selection of articles by Dulcy Mahar, who for 22 years wrote a gardening column for The Oregonian in Portland. Clearly, I haven’t been paying enough attention to gardening south of the Columbia!
Sadly, Dulcy died in 2011 after a long battle with cancer, but she continued writing up to a few weeks before her death. Fortunately her husband, Ted Mahar, has edited and published a selection of her writings. I am completely charmed by the results.
While Ted is understandably also a fan, I heartedly agree with him when he describes her columns as “…filled with solid advice, warnings, lists, ideas and experiments worth trying, the latest trends, yearnings for a change of season, and more. Whatever the subject, Dulcy’s wit glowed through. Pick a week, and you’d likely find a quotable quote.”
I would add that she had a knack of reaching out to young or inexperienced gardeners, putting them at ease, urging them not to be afraid to just go for it. She also had a love of animals, especially her cats (although one lucky dog, Hector, gets a lot of press, too). One of her Wagnerian felines is posed with her on the front cover, “helping” in the garden.
An example of her advice: “Make a list so that you can get exactly what you need when you hit the nurseries and plant sales. Oh, I could hardly say that with a straight face. I am practically rolling on the floor, and the cats and Hector the dog are looking askance. Of course, it is excellent advice. But can I follow that advice? Hardly.”
Excerpted from the Fall 2014 Arboretum Bulletin.
Rooted Cutting and Liner Care
All Van Veen Nursery rhododendrons are cutting-grown and guaranteed to be true to name. Under Kathy’s close supervision, cuttings are gathered and prepared between July and November. They are planted in open benches in traditional glass greenhouses and supplied with bottom heat and high humidity.
Since root development is the all-important consideration, most plants sold as rooted cuttings have no top growth. However, they do have large, vigorous rootballs.
PLANTING: Pot up or plant in a protected area. Use a loose, well draining medium such as one containing some bark, compost, or perlite.
WATERING: Keep soil thoroughly wet but never soggy.
PROTECTION: Shelter from heat, frost, and wind for the first 6-12 months in a protected area such as a plastic covered hoop house. Keep it open and shaded in summer. Antidessicants such as Vaporgard can be used as protection from both heat and cold. Also, antidessicants may help in preventing some fungal problems.
SHAPING: New growth is expected in 1-2 flushes the first summer. Pinching out the center bud will help the lateral buds develop thus producing a more bushy plant.
FERTILIZING: Use a liquid fertilizer 20-20-20 1T/gal. applied once every 7 to 10 days between April and August. Stop fertilizing in time to allow new growth to harden before first frost. Epsom salts, 1 T/gal., can be applied occasionally to add magnesium for better color.
WEEDING: Hand weeding is best because plants are shallow rooted. Preemergent herbicides are not recommended for these young plants.
DISEASE CONTROL: Root rot occurs in some varieties, especially in yellow flowered rhododendrons, when conditions are both hot and wet. In the Pacific Northwest this is in July and August. Well drained planting soil is essential. New biological products such as Rootshield or Actino-iron contain beneficial organisms that form protective colonies on the plant roots. These must be applied before problems are expected. Powdery mildew may be evident in some varieties and some climates. Use a systemic fungicide, such as one used for roses, so there is no need to spray leaf undersides.
PEST CONTROL: Weevils may be a problem. Watch for notching from the chewing beetles in the summer. Various insecticides may be used to control them. The grub stages of the weevil, which exists in the soil in fall, winter, and spring, does the most damage. It eats roots and underground stems and may completely girdle the plant and kill it.
JARS v56n2 – Tips for Beginners: Fertilizing Rhododendrons
Tips for Beginners: Fertilizing Rhododendrons
J. Powell Huie
Westport Point, Massachusetts
Reprinted, with revisions, from the Rosebay, published by the ARS Massachusetts Chapter
Every year in early spring and on a particularly cold day, I feel compelled to help my rhododendrons recuperate from the previous winter by giving them a generous serving of fertilizer. But after some years of experience I have concluded that in many cases I am not doing them a favor and, in some, causing considerable harm.
In general, the following practice seems to give the best results: Each spring after setting out in a lathhouse newly acquired 1- or 2-year-old plants, the majority of which come from Oregon and Washington states, are watered-in and mulched with oak leaves picked up by a lawn mower. I then scatter around each a small amount of low nitrogen fertilizer, such as Holly-tone®, a 4-6-4 mixture, recommended for acid loving plants 1 . One must resist the temptation to be overly generous, which can be deadly, particularly for the dwarf species and hybrids. One teaspoon or less around around the dwarfs will suffice. Even with this application, new leaf growth will not appear until the following spring. Keep in mind that most rhododendrons reach maturity only after up to ten years or more of normal growth, equally as true of slow-growing dwarfs as well as the larger varieties, and should not be forced to do it in less time. All that is required for normal growth of a young plant is a suitable growing medium, enough mulch to keep their roots cool in summer, enough moisture so the roots do not dry out, and the small amount of fertilizer recommended above. Thereafter, each spring again mulch and lightly fertilize. Above all be patient.
When the plants are old enough to be moved to a landscaped area, they are set out in a mixture of topsoil, bark and any available compost, and possibly a very small amount of peat moss. (The compost I use is a mixture of leaves and grass clippings put in a pile the previous year. I never bother to turn it.) A handful of superphosphate is then scattered around the plant and watered in. 2 This will carry some of the phosphate down among the roots to encourage more rapid root growth. An alternative is to mix the superphosphate into the soil prior to planting. Then, after mulching, scatter around the plant a small amount of fertilizer such as that used in the lathhouse. For the next four or five years in the late winter or spring, again add mulch and lightly fertilize. By this time the previous year’s mulch has begun to break down, and unless there is a positive sign of nitrogen, phosphate or potash deficiency, all that they then need is an annual addition of mulch, usually provided naturally from the leaves of shade trees or the plant itself.
An elepidote rhododendron having 5 or 6 inches (12.5 or 15 cm) of growth each year does not need additional nitrogen as long as the leaves are a healthy green in color. Too much nitrogen will cause lanky growth and ruin the appearance of the plant, particularly true of those located in heavy shade. Also, there is a misguided belief that in order to have a profusion of flowers on mature rhododendrons, one should make an annual application of phosphate. I followed this practice for a number of years but after discontinuing it saw little difference in the number or quality of the trusses. Phosphate is relatively insoluble, and some of that applied with the original fertilizer will remain in the soil for a number of years. Given a well-drained soil, enough decaying mulch above the roots, and sufficient moisture, an older rhododendron may be just as happy without additional help.
J. Powell Huie is a member of the Massachusetts Chapter.
JANE EDMANSON: When it comes to rhododendrons, it’s position, position, position. They have to grow in an area that is cool – overhanging branches from trees, so that they don’t get that hot afternoon sun.
Part shade, morning sun – perfect for them. They love growing in hilly areas cause the air is a little bit cooler. They don’t like to grow down by the sea because the soil is too limey.
Have a look at this. This is wonderful soil for growing perfect rhododendrons – that rich, deep, loamy soil and if you haven’t got that, you should be adding plenty of rich organic material cause that really does the job for rhododendrons.
They like moisture in their soil, but not too much cause the roots will rot. You need to keep them moist, but not too oversaturated. The drainage is important.
There’s a huge range of rhododendrons, ranging in size from really giant tree-sized ones, down to types that fit into pots. There’s small flowered rhododendrons – like that one. Gorgeous things and then look at the leaves of that. That’s a rhododendron from the Himalayas. There’s such a variety of colours – pinks, purples, yellows, reds – all sorts and there’s even a fragrant one. This isRhododendron’Fragrantissimum’ – absolutely spectacular.
Rhododendrons flower in spring and if you see the leaves looking a bit yellow, that is obviously a good time to give it some fertiliser after they’ve finished flowering – just rhododendron and azalea fertiliser. You can see the growth that they put on in spring. That’s just momentous and if you’re looking to deadhead them or prune them, grab the old flower spike and twist between your fingers – it comes off like that and that will then send up new shoots like that. You could even get in with secateurs or a hacksaw and really give it a good hard prune, right into the wood back there and it will bounce back.
Now plants do come and go in fashion, but it would be hard not to be impressed by a flower like that.
COSTA GEORGIADIS: Now our gardens generate a lot of what some people would think of as waste, but Sophie’s with me on that topic and she sees it as a resource that can be reused.
In fertile soils rhododendrons and azaleas can be grown without receiving further fertilization. In less fertile soils, a complete fertilizer designed for acid-loving plants may be applied in late winter or early spring. Be careful to use only the amounts recommended for rhododendrons and azaleas, which do not need as much fertilizer as other plants. Excessive fertilization can result in damaged roots and leaves, and some rhododendron varieties can be killed with fertilizer.
In cold climates, nitrogen fertilizer should not be applied after late June as it may promote new lush growth susceptible to winter damage. Recent research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved.
If plants are mulched with materials like fresh sawdust or wood chips, there will be a nitrogen demand caused by the decomposition of these materials, and unless nitrogen fertilizer is added, the plants are likely to show yellowish foliage and poor growth. In this case, an organic nitrogen fertilizer, such as cottonseed meal, canola meal, fish meal or blood meal, can be added. Mineral nitrogen fertilizers are associated with increased problems, such as chlorosis where leaves produce insufficient chlorophyll. Mulches other than fresh sawdust or wood chips are recommended, then you don’t have to be concerned with exactly how much fertilizer to add.
Phosphorus is required in the production of flower buds. If your soil is deficient in phosphorus and since phosphorus does not readily move through the soil, phosphorus should be incorporated into the soil at planting time if needed. Do not use phosphorus fertilizers unless a soil test indicates a deficiency. The popular perception that rhododendron and azalea flowering is enhanced by phosphorus is incorrect.
Magnesium in the form of Epsom salts is sometimes recommended for rhododendrons. Magnesium is an essential element and lack of it will cause yellowish areas between green leaf veins on older leaves. If the leaves are a solid green the addition of Epsom salts would not be useful.
Lack of iron causes much the same symptoms as lack of magnesium, but with the younger leaves showing yellowing between the green-colored veins. Iron deficiency is frequently caused by too high a soil pH, often the result of mortar or mortar building debris in the soil near the roots. A soil test should be performed to see whether high pH is a problem and if it is the soil should be acidified. For a quick but temporary solution, ferrous sulfate can be added to the soil or chelated iron can be sprayed on the foliage, but the soil pH should be corrected for long term good growth.
Calcium is also essential to good rhododendron growth. Calcium can be obtained either from gypsum or from agricultural lime. Gypsum will not raise soil pH, while lime will, therefore, lime is not generally recommended in areas with naturally alkaline soil or water.
Rhododendrons and azaleas require minute amounts of boron, manganese, zinc, molybdenum, and copper. Most of these elements are usually in the soil, but if not available, they could be the cause of poor performance.
More information about rhododendron and azalea fertilization can be found in the following resources:
Rhododendron Nutrition by Ted Van Veen
Feeding Rhododendrons Organically by Terry Richmond
Rhododendron and Azalea Fertilizing by Steve Henning
Phosphorus and Nitrogen Nutrition of Rhododendrons by George F. Ryan
Vermicomposting by Soni Cochran
Index of Contents
Pruning & Spent
Propagation & Hybridization
Insect & Disease Control